If you have the possibility of running parts of KDE on the machine that is going to generate the images, you could use the kwebdesktop program to use the khtml-Engine (the same as in Konqueror, i.e. a /very/ good one ;) to get you a rendered image of your site ...
It is intended for use as a desktop background generator, but it should work for your case just as well :

kankoku:~$ kwebdesktop --help
Usage: kwebdesktop Qt-optionsKDE-options width height file URL
(...)
Arguments:
width
Width of the image to create
height
Height of the image to create
file
Filename where to dump the output in png format.
URL
URL to open